In ink-jet printing methods, droplets of ink are directly projected onto a recording medium from very fine nozzles and allowed to adhere to the recording medium, to form characters and images. The use of ink-jet printing methods have been rapidly spread because of their various advantages such as easiness of full coloration, low cost, capability of using plain paper as the recording medium, non-contact with printed images and characters, etc.
Among such printing methods, in view of enhancing the weather resistance and water resistance of printed images and characters, an ink-jet printing method utilizing an ink containing an organic pigment as the colorant for color printing has now come to be the dominate method.
Japanese Patent Application Laid-Open (kokai) No. H10-60352 discloses a water-based pigment ink containing a pigment, a polymer dispersant, polyethyleneimine, and a water-soluble solvent, which ink is provided for the purpose of improving fixation of the pigment onto an OHP sheet, etc.
Japanese Patent Application Laid-Open (kokai) No. 2004-123865 discloses an aqueous pigment ink containing a pigment, an anionic dispersant, a cationic water-soluble polymer compound, e.g., polyethyleneimine, and an aqueous medium, which ink is provided for the purpose of improving chroma, optical density and so on in plain paper printing.
Japanese Patent Application Laid-Open (kokai) No. 2004-149633 discloses a pigment ink for ink-jet printing containing dispersed aggregates each being formed by a dispersant and a pigment particle having a size smaller than that of the aggregate, which ink is provided for the purpose of improving image chroma, etc.
Japanese Patent Application Laid-Open (kokai) No. 2006-169325 discloses a liquid for ink-jet printing containing a pigment, a water-soluble resin, a water-soluble organic solvent, and water, and describes a black pigment ink containing beaded carbon black particles of high structure type.
Japanese Patent Application Laid-Open (kokai) No. 2008-38090 discloses a water-based ink for ink-jet printing containing a colorant, and beaded or elongated metal oxide secondary particles each being formed of a plurality of primary particles thereof connected to each other, which ink is provided for the purpose of improving optical density.
Japanese Patent Application Laid-Open (kokai) No. 2006-82073 discloses a method for producing composite particles, the method including a step of adjusting the surface potential of fine particles by modifying the surface of the particles with a polymer compound; and a step of mixing a fluid containing fine particles having a positive surface potential with a fluid containing fine particles having a negative surface potential in, for example, a microchannel.
Japanese Patent Application Laid-Open (kokai) No. 2009-197097 discloses a method for forming fine particles, including a step of bringing two or more solutions (including a particle formation material solution and a poor solvent solution) into contact with one another in a mixing section of, for example, a microreactor, to thereby form fine particles; a step of bringing a coagulant into contact with the fine particles in a flow channel, to thereby form aggregates; a filtration step; and a step of redispersing the aggregates through pH adjustment, to thereby form fine particles, which method is provided for the purpose of obtaining pigment fine particles having uniform particle size.